Heretofore, as a process for obtaining a colored fine particle of a thermoplastic resin, a mechanical pulverization (or crushing) method has been utilized, which comprises, for example, coarsely crushing a colored polymer composition obtained by melt-kneading a coloring component and a polymer by using a crushing machine or other means, then pulverizing the crushed matter finely by using a jet mill or other means, and classifying the resultant by an air classifier or other means.
In such a method, however, manufacturing machines are expensive, and additionally, thus obtained particle is irregular in shape and widely varies in particle size. In order to make the polymer particle size uniform, the obtained particle should be classified. However, unusable polymer particles in size are produced in large quantities by classification, and are unfavorable from an economical viewpoint. Further, spherical fine particles are preferred from the viewpoint of blocking among particles, dispersibility, flowability or others, however, it is impossible to obtain spherical particles by mechanical pulverization (or crushing) methods.
Japanese Patent Application Laid-Open No. 275272/2002 (JP-2002-275272A, Patent Document 1) discloses a process for producing a colored fine particle polymer comprising melting a colored polymer having a coloring matter uniformly dispersed or dissolved in a thermoplastic resin, making the molten colored polymer a droplet-like fine particle in an insoluble medium having no solubility to the colored polymer, and cooling the droplet-like fine particle for solidification. In this process, however, the steps are complicated and a high temperature of the insoluble medium is required for dissolving the thermoplastic resin in the making step of the droplet-like particle. Therefore, the kind of the insoluble medium is limited to an organic solvent having a high boiling point. Moreover, in order to avoid fusing between the droplets in the cooling step, it is necessary to cool down to about 0° C., and it is impossible to stably obtain particles having a uniform particle size.
Japanese Patent Application Laid-Open No. 176065/1998 (JP-10-176065A, Patent Document 2) discloses a process for obtaining a spherical fine particle of a thermoplastic resin (a), which comprises melt-kneading the thermoplastic resin (a) to be powdered with other one or more of thermoplastic resin(s) (b) to give a polymer composition comprising the resin (a) constituting the dispersed phase and the resin (b) constituting the continuous phase, and washing the polymer composition with a solvent under conditions incapable of dissolving the resin (a) and capable of dissolving the resin (b). The document also mentions a process for adding various additives (including a pigment or a dye) to the resin (a). In this process, however, since the additive (such as a pigment or a dye) is also distributed to the resin (b) constituting the continuous phase, the additive such as a pigment cannot be effectively utilized as an agent for imparting a function to a resin and has an economical disadvantage. Moreover, in this process, it is necessary that the resin to be powdered is a resin excellent in solvent resistance and that the dispersed phase and the continuous phase are immiscible with each other. Further, it is necessary to appropriately select the combination of the resin constituting the continuous phase and the solvent depending on the kind of the resin of the dispersed phase. Therefore, not only the combination of these resins but also that of the resins and the solvent are limited to a specific one. Further, in the process of cooling the disperse composition, since resins immiscible with each other tend to induce a large phase separation, a fine particle having a given shape cannot be obtained due to reaggregation of once produced dispersed phase. Furthermore, the resin constituting the continuous phase is to be recovered in the end, or to be discarded in a dissolved state, as it is not included in the resin fine particle as a final product. However, recovery of the resin in the solution not only is very difficult but also is a caused factor of increase in the production cost of the polymer particle. Moreover, in the case of discarding the resin solution directly as a waste fluid, adverse effects on the environment cannot be ignored.                Patent Document 1: JP-2002-275272A (Claim 1)        Patent Document 2: JP-10-176065A (Claim 1, Paragraph No. [0042])        